Grease composition and bearing

ABSTRACT

The invention provides a grease composition which contains (a) a base oil, (b) a thickener, (c) a compound having an alkylthiocarbamoyl group, represented by the formula (1), (d) a hindered phenol antioxidant, and (e) a metal deactivator; 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2 , R 3  and R 4  are each independently an alkyl group having 1 to 20 carbon groups, and X represents S, S—S, S—CH 2 —S, S—CH 2 CH 2 —S or S—CH 2 CH 2 CH 2 —S.

TECHNICAL FIELD

The present invention relates to a grease composition comprising a base oil, a thickener, a particular antioxidant and a metal deactivator. In particular, the invention relates to a grease composition with improved lubrication life, suitable for use in rolling bearings operated under high temperatures, and bearings where the above-mentioned grease composition is enclosed.

BACKGROUND ART

In response to the recent demands for higher speed, energy saving, reduction of noise and minimum maintenance, the motors used in railway vehicles, automobiles, electrical machines, various kinds of industrial machines and the like are being reduced in size and weight, improved toward higher-speed rotation, higher efficiency and longer life. The tendency of motors toward smaller size and lighter weight as mentioned above decreases the amount of lubricant to be used, and the higher-speed rotation of motors tends to increase the operating temperature, which has made the operating environments of lubricant severer. Especially, in the main motor for railway vehicles, totally-enclosed motors are being used to solve the problem of noise development and meet the demand for extension of the maintenance interval. When compared with the conventional open motors, the totally-enclosed motors require more heat-resistant and longer life greases because the cooling efficiency is decreased in the totally-enclosed motors.

There are various proposals for improving the performance of grease composition.

There is proposed a grease composition which is designed to extend the lubrication life of rolling bearings under high temperatures by using a base oil, a compound having an alkylthiocarbamoyl group as the thickener, and an amine-ketone condensate antioxidant (in JP 2007-238755 A).

There is proposed a lubricant with improved fatigue life where a compound having an alkylthiocarbamoyl group and a molybdenum compound are added to the lubricant base, and an ester compound is optionally further added thereto to improve the fatigue life (e.g., in JP 03-31760 B).

There are proposed an improved abrasive-resistant extreme-pressure composition having resistance to oxidation, containing (a) methylenebis(di-n-butyldithiocarbamate) and (b) a diphenylamine derivative of tolutriazole or benzotriazole at a ratio by mass of about 4:1 to about 50:1, and a lubricant containing the above-mentioned extreme pressure composition (e.g., in JP 2005-509732 A).

There is proposed a lithium grease composition comprising (a) a lubricating oil, (b) at least one compound selected from the group consisting of lithium salts of aliphatic monocarboxylic acid having at least one hydroxyl group and 12 to 24 carbon atoms, dilithium salts of aliphatic dicarboxylic acid having 2 to 12 carbon atoms, and lithium salts of aliphatic monocarboxylic acid having 12 to 24 carbon atoms, and (c) at least one compound selected from the group consisting of 3-(N-salicyloyl)amino-1,2,4-triazole and decamethylene-dicarboxylic acid-disalicyloyl hydrazide (e.g., in JP 2936084).

However, even when those grease compositions comprising various additives as mentioned above are used in rolling bearings operated under high temperatures, the thermo-oxidative stability and the lubrication life are unsatisfactory. There has not been any grease product that can satisfy any users.

SUMMARY OF INVENTION Technical Problem

Grease compositions comprising a urea compound that contains no metal in the molecule thereof as the thickener and a synthetic oil as the base oil are commonly used for automotive auxiliaries mainly employing deep-groove radial bearings because those grease compositions are considered to be excellent in thermo-oxidative stability. However, when those grease compositions are applied to the main motor of railway vehicles where cylindrical roller bearings are indispensable, sufficient lubrication life cannot be obtained.

An object of the invention is to provide a grease composition suitable for rolling bearings under elevated temperatures and capable of exhibiting stable lubricating properties for an extended period of time. Namely, it is an object to provide a grease composition excellent in the thermo-oxidative stability and the lubrication life.

Another object of the invention is to provide a bearing where the above-mentioned grease composition is enclosed.

Solution to Problem

As a result of the extensive studies to solve the above-mentioned problems, the inventors of the invention have completed a grease composition which has improved thermo-oxidative stability and lubrication life, and is suitable for rolling bearings operated under high temperatures, and exhibits stable lubricating properties for an extended period of time by using a base oil, a thickener, particular antioxidants and a metal deactivator in combination.

Namely, the invention provides a grease composition and a bearing shown below:

1. A grease composition comprising (a) a base oil, (b) a thickener, (c) a compound having an alkylthiocarbamoyl group, represented by the formula (1), (d) a hindered phenol antioxidant, and (e) a metal deactivator;

wherein R₁, R₂, R₃ and R₄ are each independently an alkyl group having 1 to 20 carbon groups, and X represents S, S—S, S—CH₂—S, S—CH₂CH₂—S or S—CH₂CH₂CH₂—S.

2. The grease composition described in the above-mentioned item 1, wherein the hindered phenol antioxidant (d) is 2,6-di-tert-butyl-4-methylphenol.

3. The grease composition described in the above-mentioned item 1 or 2, wherein the metal deactivator (e) is decamethylenecarboxylic acid disalicyloyl hydrazide.

4. The grease composition described in any one of the above-mentioned items 1 to 3, wherein the base oil (a) is a mixture of a polyol ester oil and an alkyldiphenyl ether oil.

5. The grease composition described in any one of the above-mentioned items 1 to 4, wherein the thickener (b) is at least one selected from the group consisting of lithium soaps and lithium complex soaps.

6. The grease composition described in any one of the above-mentioned items 1 to 5, wherein the base oil (a) is contained in an amount of 50 mass % or more; the thickener (b), in an amount of 1 to 30 mass %; the compound (c) having the alkylthiocarbamoyl group represented by the formula (1), in an amount of 0.1 to 10 mass %; the hindered phenol antioxidant (d), in an amount of 0.1 to 10 mass %; and the metal deactivator (e), in an amount of 0.1 to 10 mass %, with respect to the total mass of the grease composition.

7. A bearing where the grease composition described in any one of the above-mentioned items 1 to 6 is enclosed.

Advantageous Effects of Invention

The grease composition of the present invention is remarkably advantageous because the grease composition has improved thermo-oxidative stability and lubrication life, and is suitable for use in the rolling bearings under high temperatures, and in addition, exhibits stable lubricating properties for an extended period of time. The grease composition of the invention can maintain a constant consistency and does not extremely harden or soften for an extended period of time even when used under high temperatures.

Description of Embodiments (a) Base Oil

As the base oil used in the grease composition of the invention, mineral oils, synthetic oils and mixtures thereof can be used. Examples of the synthetic oils include ester type synthetic oils such as diesters and polyol esters, synthetic hydrocarbon oils such as poly α-olefins and polybutene, ether type synthetic oils such as alkyl diphenyl ethers and polypropylene glycols, silicone type synthetic oils, fluorinated synthetic oils, and the like.

According to the invention, a mixed oil may be preferably used. A mixed oil of two or more kinds of synthetic oils, in particular, two or more ester type synthetic oils is more preferable. Especially, a mixed oil of remarkably heat-resistant polyol ester type synthetic oil and alkyldiphenyl ether type synthetic oil is most preferable.

Preferably, the base oil may have a kinematic viscosity at 40° C. of 30 to 300mm²/s. When the kinematic viscosity at 40° C. is less than 30mm²/s, the increase of evaporation loss and the decrease in oil film thickness will lower the lubricating properties and therefore the long life cannot be expected. On the other hand, when the kinematic viscosity at 40° C. exceeds 300mm²/s, the viscous resistance will increase and therefore the torque will unfavorably be increased.

The grease composition of the invention may preferably contain the base oil (a) in an amount of 50 mass % or more, and more preferably 60 mass % or more, with respect to the total mass of the grease composition.

(b) Thickener

Any thickeners are usable for the grease composition of the invention. Specific examples include soap-type thickeners such as lithium soaps and lithium complex soaps, urea type thickeners such as diurea compounds, inorganic thickeners such as organoclay and silica, organic thickeners such as PTFE, and the like.

The lithium complex soaps that can be used in the invention may preferably be lithium salts of hydroxy fatty acids (C12-24) having one or more hydroxyl groups and aliphatic dicarboxylic acids (C2-12). Examples of the above-mentioned hydroxy fatty acid include 12-hydroxystearic acid, 12-hydroxylauric acid, 16-hydroxypalmitic acid and the like. In particular, 12-hydroxystearic acid is preferred. Examples of the above-mentioned aliphatic dicarboxylic acid include azelaic acid, sebacic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, undecanedioic acid, and dodecanedioic acid. In particular, azelaic acid is preferred.

As the thickener used in the invention, soap type thickeners are preferable, and lithium soaps and lithium complex soaps are more preferable because of the excellent heat resistance. Of the lithium complex soaps, a lithium salt of 12-hydroxystearic acid and azelaic acid is most preferable. The lithium soaps and the lithium complex soaps are more advantageous over urea compounds in the invention because longer lubrication life can be ensured when applied to the main motors where the cylindrical roller bearing is indispensable.

The grease composition of the invention may preferably contain the thickener (b) in an amount of 1 to 30 mass %, and more preferably 5 to 20 mass %, with respect to the total mass of the grease composition. Too much thickener makes the resultant grease hard, which may unfavorably increase the stirring resistance. On the other hand, when the content of thickener is too small, the resultant grease becomes soft and may cause the problem of leakage.

Additives

The grease composition of the invention comprises as the antioxidants, (c) the compound having an alkylthiocarbamoyl group represented by the formula (1) (ATC), and (d) the hindered phenol antioxidant. The grease composition of the invention further comprises (e) the metal deactivator.

The compound (c) having an alkylthiocarbamoyl group used in the invention is represented by the formula (1).

In the formula (1), R₁, R₂, R₃ and R₄ are each independently a straight-chain or branched alkyl group having 1 to 20 carbon atoms, preferably a straight-chain or branched alkyl group having 1 to 8 carbon atoms. Preferably, R₁ to R₄ may be the same.

X represents S, S—S, S—CH₂—S, S—CH₂CH₂—S or S—CH₂CH₂CH₂—S. In particular, S—CH₂—S is preferable.

Examples of the compound represented by the formula (1) include bis(dimethylthiocarbamoyl)monosulfide, bis(dimethylcarbamoyl)disulfide, methylenebis(dimethyldithiocarbamate), methylenebis(di-n-butyldithiocarbamate), and the like. In particular, methylenebis(di-n-butyldithiocarbamate) is preferred.

The grease composition of the invention may preferably comprise the compound (c) having the alkylthiocarbamoyl group represented by the formula (1) in an amount of 0.1 to 10 mass %, and more preferably in an amount of 0.5 to 5 mass %, with respect to the total mass of the grease composition. When the content of the compound (c) exceeds 10 mass %, the oxidative deterioration inhibiting effect will be saturated. When the content of the compound (c) is less than 0.1 mass %, there is no significant effect even though the compound (c) is added.

As the hindered phenol antioxidant (d), at least one can be selected from the group consisting of 2,6-di-tert-butyl-4-methylphenol, pentaerythritoltetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), benzenepropanoic acid, 3,5-bis(1,1-dimethyl-ethyl)-4-hydroxy-C₇₋₉alkyl ester (side chain), and octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate. In particular, 2,6-di-tert-butyl-4-methylphenol is preferred.

The grease composition of the invention may preferably comprise the hindered phenol antioxidant (d) in an amount of 0.1 to 10 mass %, more preferably 0.5 to 5 mass %, with respect to the total mass of the grease. When the content of the component (d) exceeds 10 mass %, the effect of inhibiting oxidative deterioration will be saturated. On the other hand, when the content of the component (d) is less than 0.1 mass %, there is no significant effect even though the hindered phenol antioxidant (d) is added.

Examples of the metal deactivator (e) include oxalic acid derivatives, salicylic acid derivatives, hydrazine derivatives and the like. To be more specific, 3-(N-salicyloyl)amino-1,2,4-triazole represented by the formula (2), decamethylenecarboxylic acid disalicyloylhydrazide (MCSH) represented by the formula (3), N,N-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyl]hydrazine represented by the formula (4), isophthalic acid bis(2-phenoxypropionylhydrazide), N-formyl-N′-salicyloylhydrazine, 2,2-oxamidebis-[ethyl-3-(3,5-di-t-butyl-4-hydroxy phenyl)propionate], oxalyl-bis-benzylidene-hydrazide and the like can be used. In particular, decamethylenecarboxylic acid disalicyloylhydrazide (MCSH) represented by the formula (3) is preferred.

The grease composition of the invention may preferably comprise the metal deactivator (e) in an amount of 0.1 to 10 mass %, more preferably 0.1 to 5 mass %, with respect to the total mass of the grease composition. When the content of the component (e) exceeds 10 mass %, the metal deactivating effect will be saturated. On the other hand, when the content of the component (e) is less than 0.1 mass %, there is no significant effect even though the metal deactivator (e) is added.

In addition to the above, all the additives may be used when necessary. To be more specific, other antioxidants than those used as the essential components in the invention, for example, quinoline antioxidants, amine antioxidants, rust preventive, metal corrosion inhibitor, oilness improver, antiwear agent, extreme pressure agent, solid lubricant and the like can be used.

The grease composition of the invention may preferably comprise the base oil (a) in an amount of 50 mass % or more, more preferably 60 mass % or more; the thickener (b), in an amount of 1 to 30 mass %, more preferably 5 to 20 mass %; the compound (c) having the alkylthiocarbamoyl group represented by the formula (1), in an amount of 0.1 to 10 mass %, more preferably 0.5 to 5 mass %; the hindered phenol antioxidant (d), in an amount of 0.1 to 10 mass %, more preferably 0.5 to 5 mass %; and the metal deactivator (e), in an amount of 0.1 to 10 mass %, more preferably 0.1 to 5 mass %, with respect to the total mass of the grease composition.

Worked Penetration

The worked penetration of the grease composition according to the invention is preferably 200 to 400, more preferably 250 to 350. When the worked penetration exceeds 400, the leakage may tend to easily occur. On the other hand, when the worked penetration is less than 200, the stirring resistance will unfavorably increase.

The suitable bearings to be charged with the grease composition of the invention are rolling bearings, especially roller bearings, in particular, cylindrical roller bearings. The bearings used for the totally-enclosed motors, in particular the totally-enclosed main motors for railway vehicles are desirable. The material for bearings is not particularly limited, but preferably copper.

EXAMPLES 1. Test Greases

According to the formulations shown in Tables 1 and 2, grease compositions of Examples 1 to 7 and Comparative Examples 1 to 6 were prepared. The amount of each component means mass % based on the total mass of the grease composition.

2. Preparation Method of Greases

12-hydroxystearic acid (82.5 g) was added to each base oil (467.5 g). The resultant mixture was heated to a temperature (80 to 90° C.) where the mixture assumed a completely transparent liquid state. To this mixture, a solution prepared in advance by dissolving 11.9 g of lithium hydroxide monohydrate into 60 g of water under application of heat was added, followed by vigorous stirring to subject 12-hydroxystearic acid to a saponification reaction. Thus, a lithium salt of 12-hydroxystearic acid was formed. Subsequently, with addition of azelaic acid (26 g), the stirring was continued to obtain a uniform mixture. To this mixture, a solution prepared in advance by dissolving 11.9 g of lithium hydroxide monohydrate into 60 g of water under application of heat was added, followed by vigorous stirring to subject azelaic acid to a saponification reaction. About 60 minutes later, it was recognized by infrared spectroscopic analysis that there was no absorption of unreacted fatty acid, so that the saponification reaction was terminated. In the subsequent heating step, the reaction mixture was gradually heated to 200° C. When the temperature reached 200° C., the rest of base oil (400.2 g) was added, and the resultant mixture was cooled to room temperature on standing. Finally, the mixture was allowed to pass through a three-roll mill two times, to obtain a lithium complex grease. The mass of the final sample was set at 1000 g.

The base oil (with a kinematic viscosity of 101 mm²/s at 40° C.) was a mixture of an ether oil (alkyldiphenyl ether with kinematic viscosities of 100 mm²/s at 40° C. and 12.5 mm²/s at 100° C.) and an ester oil (pentaerythritol ester with kinematic viscosities of 102 mm²/s at 40° C. and 12.9 mm²/s at 100° C.) at a ratio by mass of 1:1.

3. Methods for Analysis and Evaluation Tests (1) Worked Penetration (JIS K2220 7)

The worked penetration of every grease composition was adjusted to 280.

(2) High-Temperature Thin Film Test

Each grease was applied to the surface of an SPCC stainless steel sheet (60 mm×80 mm×1 mm) in a thickness of 2 mm. The total acid number was determined before and after the test.

The total acid number was determined after test, i.e., after leaving the stainless sheet at a prescribed temperature (160° C.) for a prescribed duration (500 hours). The total acid number was determined in accordance with JIS K2501 5.

(3) Bearing Lubrication Life Test

A bearing charged with each test grease (5.6 g) was set in a tester. With the application of a radial load (100 kgf), the bearing was rotated at 8000 rpm while the outer ring temperature of the bearing was constantly controlled at 120° C. by use of a heater. The time duration taken for the outer ring temperature of bearing to reach 130° C. was regarded as the life time.

Test Conditions

Bearing: cylindrical roller bearing (model: NU214, with an inner diameter of 70 mm and an outer diameter of 125 mm)

Load: radial load of 100 kgf

Number of revolutions: 8000 rpm

Amount of grease enclosed in the bearing: 5.6 g

Temperature: 120° C. (thermostatted)

Evaluation criteria

-   -   Change in the total acid number (mgKOH/g)         -   o: 0.20 or less,         -   x: more than 0.20     -   Bearing lubrication life         -   o: more than 1000 hours (acceptable)         -   x: 1000 hours or less (not acceptable)     -   Overall evaluation         -   o: when the results in both tests of the change in the total             acid number and the bearing lubrication life were             acceptable.         -   x: when the results in the test of the change in the total             acid number and/or the test of the bearing lubrication life             were not acceptable.

4. Results of Evaluation Tests

The test results of the grease compositions according to Examples 1 to 7 and those of the comparative grease compositions of Comparative Examples 1 to 6 are respectively shown in Tables 1 and 2.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 (a) Base oil⁽*¹⁾ 84.35 83.40 83.85 81.80 80.50 78.85 84.80 (b) Thickener 13.0 13.0 13.0 13.0 13.0 13.0 13.0 (c) ATC⁽*²⁾ 0.5 0.5 1.0 1.0 0.5 5.0 1.0 (d) Hindered phenol 2.0 2.0 2.0 4.0 5.0 2.0 1.0 antioxidant⁽*³⁾ (e) MCSH⁽*⁴⁾ 0.1 0.1 0.1 0.1 1.0 0.1 0.1 Quinoline antioxidant⁽*⁵⁾ — — — — — — — Amine antioxidant⁽*⁶⁾ — 1.0 — — — 1.0 — Copper corrosion inhibitor 0.05 — 0.05 0.1 — 0.05 0.1 Total acid number (mgKOH/g) 0.06 0.05 0.08 0.04 0.03 0.06 0.07 before high temperature thin film test Total acid number (mgKOH/g) 0.16 0.11 0.20 0.21 0.13 0.18 0.15 after high temperature thin film test Change in total acid number 0.10 0.06 0.12 0.17 0.10 0.12 0.08 (mgKOH/g) Evaluation of change in total ∘ ∘ ∘ ∘ ∘ ∘ ∘ acid number Bearing lubrication life test 1670 1530 1600 1410 1700 1450 1380 (120° C.) (hours) Evaluation of bearing ∘ ∘ ∘ ∘ ∘ ∘ ∘ lubrication life test Overall evaluation ∘ ∘ ∘ ∘ ∘ ∘ ∘ ⁽*¹⁾Mixture of an ether oil (alkyldiphenyl ether with kinematic viscosities of 100 mm²/s at 40° C. and 12.5 mm²/s at 100° C.) and an ester oil (pentaerythritol ester with kinematic viscosities of 102 mm²/s at 40° C. and 12.9 mm²/s at 100° C.) at a ratio by mass of 1:1. The mixture has a kinematic viscosity of 101 mm²/s at 40° C. ⁽*²⁾Methylenebis(di-n-butyldithiocarbamate) ⁽*³⁾Hindered phenol antioxidant: 2,6-di-tert-butyl-4-methylphenol ⁽*⁴⁾Decamethylenecarboxylic acid disalicyloyl hydrazide ⁽*⁵⁾2,2,4-trimethyl-1,2-dihydroquinoline polymeric product ⁽*⁶⁾p,p′-dioctyldiphenylamine

TABLE 2 Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 (a) Base oil⁽*¹⁾ 86.35 84.85 84.45 84.35 84.35 82.35 (b) Thickener 13.0 13.0 13.0 13.0 13.0 13.0 (c) ATC⁽*²⁾ 0.5 — 0.5 0.5 0.5 0.5 (d) Hindered phenol — 2.0 2.0 — — — antioxidant⁽*³⁾ (e) MCSH⁽*⁴⁾ 0.1 0.1 — 0.1 1.0 0.1 Quinoline antioxidant⁽*⁵⁾ — — — 2.0 — 2.0 Amine antioxidant⁽*⁶⁾ — — — — 2.0 2.0 Copper corrosion inhibitor 0.05 0.05 0.05 0.05 0.05 0.05 Total acid number 0.08 0.03 0.04 0.04 0.07 0.03 (mgKOH/g) before high temperature thin film test Total acid number 1.29 1.12 0.89 0.75 1.50 1.38 (mgKOH/g) after high temperature thin film test Change in total acid number 1.21 1.09 0.85 0.71 1.43 1.35 (mgKOH/g) Evaluation of change in total x x x x x X acid number Bearing lubrication life test 730 850 700 650 780 810 (120° C.) (hours) Evaluation of bearing x x x x x x lubrication life test Overall evaluation x x x x x x ⁽*¹⁾Mixture of an ether oil (alkyldiphenyl ether with kinematic viscosities of 100 mm²/s at 40° C. and 12.5 mm²/s at 100° C.) and an ester oil (pentaerythritol ester with kinematic viscosities of 102 mm²/s at 40° C. and 12.9 mm²/s at 100° C.) at a ratio by mass of 1:1. The mixture has a kinematic viscosity of 101 mm²/s at 40° C. ⁽*²⁾Methylenebis(di-n-butyldithiocarbamate) ⁽*³⁾Hindered phenol antioxidant: 2,6-di-tert-butyl-4-methylphenol ⁽*⁴⁾Decamethylenecarboxylic acid disalicyloyl hydrazide ⁽*⁵⁾2,2,4-trimethyl-1,2-dihydroquinoline polymeric product ⁽*⁶⁾p,p′-dioctyldiphenylamine

The grease compositions of Examples 1 to 7 comprising all the components (a) to (e) showed little change in the total acid number and excellent thermo-oxidative stability, and also met the standards of the bearing lubrication life, and were therefore regarded as acceptable in the overall evaluation.

The grease compositions of Comparative Examples 1, 2 and 3 where the components (d), (c) and (e) were not contained respectively showed considerable changes in the total acid number and poor thermo-oxidative stability, and also short bearing lubrication life.

The grease compositions of Comparative Examples 4 and 5 where the quinoline antioxidant and the amine antioxidant were respectively contained instead of the component (d) used in Example 1 also showed considerable changes in the total acid number and poor thermo-oxidative stability, and also short bearing lubrication life.

The grease composition of Comparative Example 6 where both the quinoline antioxidant and the amine antioxidant were contained instead of the component (d) used in Example 1 also showed considerable changes in the total acid number and poor thermo-oxidative stability, and also short bearing lubrication life. 

1. A grease composition comprising (a) a base oil, (b) a thickener, (c) a compound having an alkylthiocarbamoyl group, represented by the formula (1), (d) a hindered phenol antioxidant, and (e) a metal deactivator;

wherein R₁, R₂, R₃ and R₄ are each independently an alkyl group having 1 to 20 carbon groups, and X represents S, S—S, S—CH₂—S, S—CH₂CH₂—S or S—CH₂CH₂CH₂—S.
 2. The grease composition of claim 1, wherein the hindered phenol antioxidant (d) is 2,6-di-tert-butyl-4-methylphenol.
 3. The grease composition of claim 1, wherein the metal deactivator (e) is decamethylenecarboxylic acid disalicyloyl hydrazide.
 4. The grease composition of claim 1, wherein the base oil (a) is a mixture of a polyol ester oil and an alkyldiphenyl ether oil.
 5. The grease composition of claim 1, wherein the thickener (b) is at least one selected from the group consisting of lithium soaps and lithium complex soaps.
 6. The grease composition of claim 1, wherein the base oil (a) is contained in an amount of 50 mass % or more; the thickener (b), in an amount of 1 to 30 mass %; the compound (c) having the alkylthiocarbamoyl group represented by the formula (1), in an amount of 0.1 to 10 mass %; the hindered phenol antioxidant (d), in an amount of 0.1 to 10 mass %; and the metal deactivator (e), in an amount of 0.1 to 10 mass %, with respect to the total mass of the grease composition.
 7. A bearing where the grease composition of claim 1 is enclosed. 